A teddy bear rests at the post of a sign near the entrance to the parking lots for the Pittsburgh Zoo and PPG Aquarium on Monday, Nov. 5, 2012, in Pittsburgh. Zoo officials said a young boy was killed after he fell into the exhibit that was home to a pack of African painted dogs, who pounced on the boy and mauled him on Sunday, Nov. 4, 2012. (AP Photo/Keith Srakocic)

Photo: Keith Srakocic, Associated Press

A teddy bear rests at the post of a sign near the entrance to the...

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Barbara Baker, CEO and president of the Pittsburgh Zoo and PPG Aquarium, gestures to indicate the slope of a railing that is in the observation area of the African Painted Dog exhibit during a news conference on Monday, Nov. 5, 2012, in Pittsburgh. Zoo officials said a young boy was killed after he fell into the exhibit and the dogs mauled him on Sunday, Nov. 4, 2012. (AP Photo/Keith Srakocic)

A 2-year-old boy who fell into an exhibit of African wild dogs was killed by the animals, not the fall, the head of the Pittsburgh Zoo said Monday, adding a horrific coda to a tragedy that has devastated staff and shocked patrons.

The boy's mother had picked him up and put him on top of a railing at the edge of a viewing deck Sunday when he lost his balance and fell, said Barbara Baker, president of the Pittsburgh Zoo and PPG Aquarium. There was a safety net below the railing, but it failed to catch him, and the boy dropped more than 10 feet into the enclosure, she said.

Nearby staff responded "within seconds" on Sunday but quickly determined that the dog attack was fatal and didn't send handlers into the enclosure to intervene, Baker said.

Instead, the dogs were recalled into an indoor enclosure, though four of the 11 lingered near the boy even after blank anesthetic darts, used out of an abundance of caution for the boy's safety, were fired to shoo them away. One of the dogs, which are endangered, was fatally shot by police.

Baker said she had been informed by the Allegheny County medical examiner that an autopsy determined the boy survived the plunge. The medical examiner's office has not yet publicly confirmed its findings or released the boy's name.